The present invention generally relates to optical transmission of information and more particularly to an optical transmission module for use in optical telecommunications, optical information processings, optical interconnections, and the like.
Optical transmission modules are used extensively in optical telecommunication systems for interconnecting and/or switching optical paths. Further, such optical transmission modules are used also for various optical information processings.
FIG. 1 shows the construction of a conventional optical reception module disclosed by Moriya et al., in IEICE, spring meeting 1992, C-269, p.4-311.
Referring to FIG. 1, the optical reception module includes a carrier 11, a lens array 12, a flat optical fiber cable 13, a photodiode array 14, an adjustment block 14, and an alignment member 16, wherein the photodiode array 14 is carried by the carrier 11, and the carrier 11 is held on an adjustment block 15. The alignment member 16 is formed of a silicon block having V-grooves for holding optical fibers that are arranged to form the flat optical fiber cable 13. Further, the lens array 12 carries thereon a plurality of monolithic microlenses for focusing the optical beams in the optical fibers forming the cable 13, upon corresponding photodiodes that form the photodiode array 14.
The conventional optical reception module of FIG. 1, having such a construction for mounting the optical elements and the optical fibers separately on an adjustment block, has a problem in that one has to achieve a complex optical alignment process for each of the modules. Further, the optical reception module having such a construction tends to have a large size.
FIGS. 2A and 2B show another conventional optical module used for optical path conversion disclosed in the Japanese Laid-open Patent Publication 4-208905, wherein the optical module achieves an efficient optical coupling between an optical semiconductor device and an optical waveguide with a reduced number of parts and correspondingly simple process for alignment. Further, the optical module of FIGS. 2A and 2B has an advantageous feature of reduced backward propagation of optical beam. It should be noted that the device of FIGS. 2A and 2B achieves the desired optical coupling by using the exposed crystal surfaces defining a depression in a semiconductor substrate, for a reflection surface.
FIG. 2A shows the construction of a reflector block 49 in an enlarged scale, while FIG. 2B shows the mounting of a photodiode block 48 on the reflector block 49, together with a fiber connector 50 provided at an end of an optical fiber 53.
Referring to FIGS. 2A and 2B, the reflector block 49 includes a silicon substrate 41 covered by an oxide film 42, wherein a V-shaped recess 45 is formed on the surface of the oxide film 42 as well as in the silicon substrate 41 by an anisotropic etching process, such that the recess 45 extends to the interior of the substrate 41. More specifically, a groove is formed on the oxide film 42 in correspondence to the recess 45 to be formed, by employing a suitable masking process, so as to expose the surface of the silicon substrate 41, followed by an anisotropic etching process applied to the exposed surface of the silicon substrate 41. Thereby, the recess 45 is defined by a pair of side walls 43 and 44 acting as reflecting surfaces. Typically, the side walls 43 and 44 are formed of a crystal surface having an {111} orientation.
The reflector block 49 thus formed is then provided with a marker 47 on the surface thereof, and the photodiode block 48 is mounted upon the surface of the reflector block 49 according to a flip-chip process. By providing an interconnection pattern 46 on the surface of the oxide film 42, it is possible to achieve the mounting of the photodiode block 48 and the interconnection in a single step.
The optical connection between the fiber connector 50 and the reflector block 49 is achieved by abutting both blocks with each other with a high precision alignment achieved by a pair of alignment pins 51 that engage with the side walls of the block 48. Thus, the alignment pins are provided with a high precision with respect to the end of the optical fiber 53 held in the connector 50 and designated in FIG. 2B by a numeral 52. Upon mounting of the photodiode block 48, an optical coupling is achieved between the photodiode in the photodiode block 48 and the optical fiber 52 in the connector 50 by way of the reflection surfaces 43 and 44 that form the recess 45.
The optical module of FIGS. 2A and 2B thus constructed, on the other hand, has a drawback, associated with use of the polygonal mirror surfaces defining the recess 45, of degraded optical performance such as aberration when focusing the optical beam in the optical connector 50 upon the photodiode block 48. It should be noted that, because of the use of the polygonal reflection surface in the block 49, the optical beam emitted at the end 52 of the optical fiber 53 does not converge upon the corresponding photodiode. Further, the optical module has a problem of mechanical strength as well as cost associated with the use of fragile and expensive silicon substrate.